^o o Scientific Proceedings^ Royal Dublin Society, 



beautiful specimen — consisting of three interpenetrating crystals. 

 Their faces are very perfect, and extinction for each crystal is 

 parallel to its longitudinal axis. These are soluble in hydro- 

 fluoric acid after long immersion. Just such crystals are depicted 

 in the Mineralogie Micrographique of MM. Fouque and Levy, as 

 extracted from the lavas of Santorin of 1866. 



The monoclinic pyroxene, Augite, which so closely resembles the 

 hypersthene in appearance will be found to be only feebly or not 

 at all polychroic. Extinction occurs at some 30 to 37 degrees with 

 the principal axis. This angle is stated to vary with the amount 

 of iron present in the mineraL It is never so low as to confuse it 

 with hornblende. 



In general its crystals are of a brighter green colour than the 

 greenest of the hypersthene. They, also, are slowly soluble in hydro- 

 fluoric acid. Crystals of hypersthene 4 mms. long were picked 

 up : the augite rarely was found above 1 mm. in length. Similar 

 crystals fell in Norway in a volcanic ash from Iceland (Fouque 

 and Levy, op. cit.). 



Magnetite occurs not alone abundantly as inclusions in the 

 various crystals, but is plentiful as minute octahedrons exhibiting 

 the usual twin forms with octahedral composition face. It shows 

 a fine black, splendant, metallic lustre ; occasionally bluish and 

 even like that of anthracite. An electro-magnet enclosed in a long 

 test tube was found of much service in removing it from solutions, 

 and also in isolating the pyroxene containing this mineral. 



Iron Pyrites is present in such characteristic forms as to leave 

 little room for doubt as to its identity. It will be observed not 

 alone in isolated fragments, little aggregations of cubes showing 

 the striations at right angles on adjacent faces, and exhibiting the 

 brassy lustre of pyrites, but also, imbedded in and protruding from 

 the surface of some of the rough amorphous fragments scattered 

 through the ash. In good light the faces of these crystals are seen 

 to be generally pentagonal, and the crystals in fact to be principally 

 pentagonal dodecahedrons. 



It was thought that this substance might be foreign to the real 

 constitution of the ash. The discovery, however, of a crystal with 

 the lustre and appearance of pyrites imbedded deeply in the in- 

 terior of a feldspar rendered it impossible to suppose this mineral 

 to be merely blown up from local rocks, and so intermingled with 



